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Abstract:

Provided is a cross-linkable fluororubber composition capable of giving a
fluororubber cross-linked molded article that has excellent mechanical
strength and low friction properties. The cross-linkable fluororubber
composition includes a coagulum obtained by co-coagulating a fluororubber
(A) and a fluororesin (B).

Claims:

1. A cross-linkable fluororubber composition comprising a coagulum
obtained by co-coagulating a fluororubber (A) and a fluororesin (B).

3. The cross-linkable fluororubber composition according to claim claim
1, wherein the fluororubber (A) includes a copolymerization unit derived
from a cross-linking-site-imparting monomer.

4. The cross-linkable fluororubber composition according to claim 1,
wherein the fluororesin (B) is at least one selected from the group
consisting of ethylene/tetrafluoroethylene copolymers,
tetrafluoroethylene/hexafluoropropylene copolymers,
tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymers,
tetrafluoroethylene/vinylidene fluoride/hexafluoropropylene copolymers,
polyvinylidene fluoride, and chlorotrifluoroethylene/tetrafluoroethylene
copolymers.

5. The cross-linkable fluororubber composition according to claim claim
1, wherein a mass ratio (A)/(B) of the fluororubber (A) to the
fluororesin (B) is 60/40 to 97/3.

6. A fluororubber molded article obtained by cross-linking the
cross-linkable fluororubber composition according to claim 1.

7. A method for producing a fluororubber molded article, comprising (I) a
step of obtaining the cross-linkable fluororubber composition according
to claim 1 by co-coagulating the fluororubber (A) and the fluororesin
(B); (II) a molding and cross-linking step of obtaining a cross-linked
molded article by molding and cross-linking the cross-linkable
fluororubber composition; and (III) a heat-treatment step of obtaining a
fluororubber molded article by heating the cross-linked molded article at
a temperature not lower than a melting point of the fluororesin (B).

8. A fluororubber molded article obtained by the production method
according to claim 7.

9. The fluororubber molded article according to claim 6, wherein the
fluororubber molded article is a sealing material.

10. The fluororubber molded article according to claim 6, wherein the
fluororubber molded article is a slide member.

11. The fluororubber molded article according to claim 6, wherein the
fluororubber molded article is a non-adhesive member.

12. The fluororubber molded article according to claim 6, having water
repellency and oil repellency on a surface.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a cross-linkable fluororubber
composition, a molded article obtained by cross-linking the
cross-linkable fluororubber composition, and a method for producing the
molded article. These are suitable as various types of sealing materials,
slide members, non-adhesive members, and members having water repellency
and oil repellency on the surface.

BACKGROUND ART

[0002] Fluororubbers have excellent chemical resistance, solvent
resistance, and heat resistance, and are widely used in various fields
such as automobile industries, semiconductor industries, and chemical
industries. In the automobile industries, for example, fluororubbers are
used as hoses, sealing materials and the like used for engines and
peripheral devices thereof, automatic transmissions, fuel systems and
peripheral devices thereof, and the like.

[0003] In some cases, however, fluororubbers such as
propylene-tetrafluoroethylene copolymer rubbers embrittle at low
temperatures. Patent Document 1 suggests a method which solves such a
problem by blending an ethylene-tetrafluoroethylene copolymer resin
[ETFE] having a melting point of 240° C. to 300° C.,
melt-kneading the mixture, and irradiation cross-linking or peroxide
cross-linking the mixture.

[0004] Patent Document 2 also teaches a method of producing a cross-linked
rubber having better hot strength by pressure cross-linking a
fluororubber composition including a fluororubber (a vinylidene fluoride
[VdF] rubber), a fluororesin [ETFE], and a fluorine-containing
thermoplastic elastomer (at 160° C. for 10 minutes), and further
cross-linking the composition in an oven (at 180° C. for 4 hours).

[0005] These Patent Documents do not mention the surface properties,
particularly the friction characteristics, of the cross-linked rubber.
This is because rubbers naturally have a high coefficient of friction
because of the elastomeric properties.

[0006] In the fields of sealing materials or other products, suggested
methods of reducing the coefficient of friction while taking advantage of
the characteristics of rubber include a method of laminating, for
example, a fluororesin (or a fluororesin fibrous layer) on the surface of
the rubber (Patent Documents 3 and 4), and a method of forming a coating
film of a fluororesin on the surface of the rubber (Patent Document 5).
[0007] Patent Document 1: JP 50-32244 A [0008] Patent Document 2: JP
6-25500 A [0009] Patent Document 3: JP 7-227935 A [0010] Patent Document
4: JP 2000-313089 A [0011] Patent Document 5: JP 2006-292160 A

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

[0012] In the case of forming a fluororesin layer on the surface of the
rubber by lamination or coating, the major aim is to increase the
adhesion at the interface between the fluororubber and the fluororesin.
The current state of the art techniques, however, have difficulties in
achieving such an aim.

[0013] The present invention aims to provide a cross-linkable fluororubber
composition capable of giving a fluororubber molded article having high
mechanical strength and low friction properties, a molded article
obtainable by cross-linking the composition, and a method for producing
the molded article.

Means for Solving the Problems

[0014] The present invention has been completed upon unexpected finding
that a fluororubber molded article having high mechanical strength and
low coefficient of friction can be produced by cross-linking a
cross-linkable fluororubber composition obtained by co-coagulating a
fluororubber and a fluororesin, and then heat-treating the composition
under specific conditions, differently from the conventional lamination
or coating method.

[0015] That is, the present invention relates to a cross-linkable
fluororubber composition containing a coagulum obtained by co-coagulating
a fluororubber (A) and a fluororesin (B).

[0017] A mass ratio (A)/(B) of the fluororubber (A) to the fluororesin (B)
is preferably 60/40 to 97/3.

[0018] The present invention also relates to a fluororubber molded article
obtained by cross-linking the cross-linkable fluororubber composition.

[0019] The present invention also relates to a method for producing a
fluororubber molded article, including

[0020] (I) a step of obtaining the cross-linkable fluororubber composition
by co-coagulating the fluororubber (A) and the fluororesin (B);

[0021] (II) a molding and cross-linking step of obtaining a cross-linked
molded article by molding and cross-linking the cross-linkable
fluororubber composition; and

[0022] (III) a heat-treatment step of obtaining a fluororubber molded
article by heating the cross-linked molded article at a temperature not
lower than a melting point of the fluororesin (B).

[0023] The present invention also relates to a fluororubber molded article
obtained by the above production method.

[0024] The fluororubber molded article can be suitably used as a sealing
material, a slide member, or a non-adhesive member.

[0025] The present invention also relates to a fluororubber molded article
having water repellency and oil repellency on a surface.

Effect of the Invention

[0026] The present invention can provide a fluororubber molded article
having high mechanical strength, low friction properties, non-adhesion,
water repellency and oil repellency on a surface. The fluororubber molded
article of the present invention is useful as a sealing material, a slide
member, a non-adhesive member, or a member having water repellency and
oil repellency on the surface.

Modes for Carrying Out the Invention

[0027] The cross-linkable fluororubber composition of the present
invention contains a coagulum obtained by co-coagulating a fluororubber
(A) and a fluororesin (B).

[0028] The cross-linkable fluororubber composition of the present
invention, containing the co-coagulated fluororubber (A) and fluororesin
(B), is expected to have the fluororubber (A) and the fluororesin (B)
uniformly dispersed therein. Hence, cross-linking the cross-linkable
fluororubber composition and heat-treating the cross-linked article under
specific conditions are considered to give a fluororubber molded article
which has low friction properties as well as high mechanical strength.

[0029] Examples of the method for the above co-coagulation include (i) a
method of mixing an aqueous dispersion of the fluororubber (A) and an
aqueous dispersion of the fluororesin (B), and then coagulating the
mixture; (ii) a method of mixing the powder of the fluororubber (A) into
an aqueous dispersion of the fluororesin (B), and then coagulating the
mixture; and (iii) a method of mixing the powder of the fluororesin (B)
into an aqueous dispersion of the fluororubber (A), and then coagulating
the mixture.

[0030] The method (i) is preferable as the co-coagulation method from the
viewpoint of particularly uniform dispersion of the resins. Particularly,
the fluororubber (A) and the fluororesin (B) are preferably obtained by
mixing an aqueous dispersion of the fluororubber (A) and an aqueous
dispersion of the fluororesin (B), coagulating the mixture, recovering
the coagulum, and optionally drying the coagulum.

(A) Fluororubber

[0031] The fluororubber (A) is produced from an amorphous polymer that has
fluorine atoms bonded to carbon atoms constituting the main chain, and
has rubber elasticity. The fluororubber (A) may be produced from one kind
of polymer, or may be produced from two or more kinds of polymers.

[0033] The fluororubber (A) is preferably a copolymer containing a
vinylidene fluoride unit, or a tetrafluoroethylene (TFE)/propylene (P)
copolymer.

[0034] The fluororubber containing a vinylidene fluoride (VdF) unit
(hereinafter, such a fluororubber is also referred to as a "VdF
fluororubber") is described hereinbelow. The VdF fluororubber is a
fluororubber at least containing a copolymerization unit derived from
vinylidene fluoride.

[0035] The copolymer containing a VdF unit is preferably a copolymer
containing a VdF unit and a copolymerization unit (excluding the VdF
unit) derived from a fluorine-containing ethylenic monomer. The copolymer
containing a VdF unit preferably further contains a copolymerization unit
derived from a monomer copolymerizable with VdF and a fluorine-containing
ethylenic monomer.

[0036] The copolymer containing a VdF unit preferably contains 30 to 85
mol % of the VdF unit and 70 to 15 mol % of the copolymerization unit
derived from a fluorine-containing ethylenic monomer, and more preferably
contains 30 to 80 mol % of the VdF unit and 70 to 20 mol % of the
copolymerization unit derived from a fluorine-containing ethylenic
monomer. The copolymerization unit derived from a monomer copolymerizable
with VdF and a fluorine-containing ethylenic monomer preferably
constitutes 0 to 10 mol % of the total amount of the VdF unit and the
copolymerization unit derived from a fluorine-containing ethylenic
monomer.

[0037] Examples of the fluorine-containing ethylenic monomer include
fluorine-containing monomers such as TFE, CTFE, trifluoroethylene, HFP,
trifluoropropylene, tetrafluoropropylene, pentafluoropropylene,
trifluorobutene, tetrafluoroisobutene, perfluoro(alkyl vinyl ether)
(hereinafter, also referred to as PAVE), and vinyl fluoride. Among these,
at least one selected from the group consisting of TFE, HFP, and PAVE is
preferable.

[0038] The PAVE is preferably at least one selected from the group
consisting of compounds represented by formula (1):

CF2═CFO(CF2CFY1O)p--(CF2CF2CF2O).-
sub.q--Rf (1)

(wherein Y1 represents F or CF3, Rf represents a C1 to C5
perfluoroalkyl group, p represents an integer of 0 to 5, and q represents
an integer of 0 to 5), and compounds represented by formula (2):

CFX═CXOCF2OR1 (2)

(wherein X represents H, F, or CF2, and R1 represents a
straight chain or branched C1 to C6 fluoroalkyl group or a
C5 or C6 cyclic fluoroalkyl group).

[0039] R1 in formula (2) may be a fluoroalkyl group containing one or
two atoms selected from the group consisting of H, Cl, Br, and I.

[0040] The PAVE is preferably perfluoro(methyl vinyl ether) or
perfluoro(propyl vinyl ether), and is more preferably perfluoro(methyl
vinyl ether). Each of these may be used alone or in any combination.

[0041] Examples of the monomer copolymerizable with VdF and a
fluorine-containing ethylenic monomer include ethylene, propylene, and
alkyl vinyl ether.

[0042] Specific preferable examples of such a copolymer containing a VdF
unit include one or two or more copolymers such as VdF/HFP copolymers,
VdF/HFP/TFE copolymers, VdF/CTFE copolymers, VdF/CTFE/TFE copolymers,
VdF/PAVE copolymers, VdF/TFE/PAVE copolymers, VdF/HFP/PAVE copolymers,
and VdF/HFP/TFE/PAVE copolymers. Among these copolymers containing a VdF
unit, VdF/HFP copolymers and VdF/HFP/TFE copolymers are particularly
preferable from the viewpoints of heat resistance, compression set,
processability, and cost.

[0043] The VdF/HFP copolymer preferably has a molar ratio VdF/HFP of 45 to
85/55 to 15, more preferably 50 to 80/50 to 20, and still more preferably
60 to 80/40 to 20.

[0044] The VdF/HFP/TFE copolymer preferably has a molar ratio VdF/HFP/TFE
of 40 to 80/10 to 35/10 to 35.

[0045] The VdF/PAVE copolymer preferably has a molar ratio VdF/PAVE of 65
to 90/10 to 35.

[0046] The VdF/TFE/PAVE copolymer preferably has a molar ratio
VdF/TFE/PAVE of 40 to 80/3 to 40/15 to 35.

[0047] The VdF/HFP/PAVE copolymer preferably has a molar ratio
VdF/HFP/PAVE of 65 to 90/3 to 25/3 to 25.

[0048] The VdF/HFP/TFE/PAVE copolymer preferably has a molar ratio
VdF/HFP/TFE/PAVE of 40 to 90/0 to 25/0 to 40/3 to 35, and more preferably
40 to 80/3 to 25/3 to 40/3 to 25.

[0049] The fluororubber (A) is alternatively preferably a TFE/P copolymer.
The TFE/P copolymer preferably contains a tetrafluoroethylene unit, a
propylene unit, and a repeating unit derived from another monomer as an
optional component copolymerizable with tetrafluoroethylene and
propylene. More preferably, the TFE/P copolymer contains 90 to 100 mol %
of the tetrafluoroethylene unit and the propylene unit in total, and 10
to 0 mol % of the repeating unit derived from another monomer.

[0050] Here, another monomer is not particularly limited as long as it is
a monomer copolymerizable with the tetrafluoroethylene unit and the
propylene unit, and is preferably vinylidene fluoride (VdF).

[0051] The fluororubber (A) is also alternatively preferably a copolymer
containing a copolymerization unit derived from a
cross-linking-site-imparting monomer. Examples of the
cross-linking-site-imparting monomer include iodine-containing monomers
such as perfluoro(6,6-dihydro-6-iodo-3-oxa-1-hexene) and
perfluoro(5-iodo-3-oxa-1-pentene) described in JP 5-63482 B and JP
7-316234 A, bromine-containing monomers described in JP 4-505341 A, cyano
group-containing monomers, carboxyl group-containing monomers, and
alkoxycarbonyl group-containing monomers described in JP 4-505345 A and
JP 5-500070 A. Among these cross-linking-site-imparting monomers, cyano
group-containing monomers are preferable.

[0052] Examples of the cyano group-containing monomer include cyano
group-containing monomers represented by the following formulas (3) to
(20), and each of these may be used alone or in any combination.

CY22═CY2(CF2)n--CN (3)

(wherein Y2 is a hydrogen atom or a fluorine atom, and n is an
integer of 1 to 8)

CF2═CFCF2Rf1--CN (4)

(wherein Rf1 is --(OCF2)n-- or
--(OCF(CF3))n--, and n is an integer of 0 to 5)

CF2═CFCF2(OCF(CF3)CF2)m(OCH2CF2CF-
2)nOCH2OF2--CN (5)

(wherein m is an integer of 0 to 5 and n is an integer of 0 to 5)

CF2═CFCF2(OCH2CF2CF2)m(OCF(CF3)CF-
2)nOCF(CF3)--CN (6)

(wherein m is an integer of 0 to 5 and n is an integer of 0 to 5)

CF2═CF(OCF2CF(CF3))mO(CF2)n--CN (7)

(wherein m is an integer of 0 to 5 and n is an integer of 1 to 8)

CF2═CF(OCF2CF(CF3))m--CN (8)

(wherein m is an integer of 1 to 5)

CF2═CFOCF2(CF(CF3)OCF2)nCF(--CN)CF3
(9)

(wherein n is an integer of 1 to 4)

CF2═CFO(CF2)nOCF(CF3)--CN (10)

(wherein n is an integer of 2 to 5)

CF2═CFO(CF2)n--(C6H4)--CN (11)

(wherein n is an integer of 1 to 6)

CF2═CF(OCF2CF(CF3))nOCF2CF(CF3)--CN
(12)

(wherein n is an integer of 1 or 2)

CH2═CFCF2O(CF(CF3)CF2O)nCF(CF3)--CN
(13)

(wherein n is an integer of 0 to 5)

CF2═CFO(CF2CF(CF3)O)m(CF2)n--CN (14)

(wherein m is an integer of 0 to 5 and n is an integer of 1 to 3)

CH2═CFCF2OCF(CF3)OCF(CF3)--CN (15)

CH2═CFCF2OCH2CF2--CN (16)

CF2═CFO(CF2CF(CF3)O)mCF2CF(CF3)--CN
(17)

(wherein m is an integer not smaller than 0)

CF2═CFOCF(CF3)CF2O(CF2)n--CN (18)

(wherein n is an integer not smaller than 1)

CF2═CFOCF2OCF2CF(CF3)OCF2--CN (19)

CF2═CFOCF(CF3)CF2OCF2CF2--CN (20)

[0053] Among these, the cyano group-containing monomer represented by
formula (7), (14), or (20) is preferable from the viewpoints of
copolymerization properties and vulcanization properties, and
CF2═CFOCF2CF(CF3)OCF2CF2CN,
CF2═CFO(CF2)5CN, or
CF2═CFOCF(CF3)CF2OCF2CF2--CN is more
preferable.

[0054] In the case that the fluororubber (A) contains a copolymerization
unit derived from the cyano group-containing monomer, the cyano group is
cyclotrimerized and thus triazine cross-linking is allowed to proceed.

[0055] The copolymerization unit derived from a cyano group-containing
monomer constitutes preferably 0.1 to 5 mol %, and more preferably 0.3 to
3 mol % of the total amount of the VdF unit and the copolymerization unit
derived from a fluorine-containing ethylenic monomer, from the viewpoints
of good cross-linking characteristics and good heat resistance.

[0056] The fluororubber (A) is also preferably one having an iodine atom
or a bromine atom at an end of the main chain thereof. A fluororubber
having an iodine atom or a bromine atom at an end of the main chain
thereof can be produced by triggering emulsion polymerization of monomers
with a radical initiator in an aqueous medium in the presence of a
halogen compound and in the substantial absence of oxygen.

[0057] A typical compound used as the halogen compound may be, for
example, a compound represented by the following formula:

R2IxBry

(wherein x and y each are an integer of 0 to 2 and satisfy
1≦x+y≦2; and R2 is a saturated or unsaturated C1 to
C16 fluorohydrocarbon or chlorofluoro hydrocarbon group, or a C1 to C3
hydrocarbon group, and may contain an oxygen atom).

[0059] Among these, 1,4-diiodoperfluorobutane or diiodomethane are
preferable from the viewpoints of polymerization reactivity,
cross-linking reactivity, and easy availability.

[0060] The fluororubber (A) preferably has a Mooney viscosity
(ML1+10(121° C.)) of 5 to 140, more preferably 10 to 120, and
still more preferably 20 to 100, from the viewpoint of good
processability.

[0061] The fluororubber (A) preferably has a number average molecular
weight of 20,000 to 1,200,000, more preferably 30,000 to 300,000, and
still more preferably 50,000 to 200,000.

[0062] The fluororubber (A) used in the present invention is preferably a
fluororubber having a fluorine content of not lower than 50% by mass,
more preferably a fluororubber having a fluorine content of not lower
than 60% by mass, and still more preferably a fluororubber having a
fluorine content of not lower than 65% by mass. The maximum fluorine
content is not particularly limited, and is preferably not higher than
74% by mass. Too low a fluorine content tends to bring inferior chemical
resistance, inferior fuel resistance, and inferior low fuel
penetrability.

[0063] The cross-link system for the fluororubber (A) can be selected
according to the application. Examples of the cross-linking system
include peroxide cross-linking systems, polyol cross-linking systems,
polyamine cross-linking systems, oxazole cross-linking systems, imidazole
cross-linking systems, thiazole cross-linking systems, triazine
cross-linking systems, and irradiation cross-linking systems. The
cross-linkable fluororubber composition of the present invention may
contain a cross-linking agent or ammonia-producing compound used in each
cross-linking system.

[0064] The peroxide cross-linking can be performed when a
peroxide-cross-linkable fluororubber and an organic peroxide as the
cross-linking agent are used.

[0065] The peroxide-cross-linkable fluororubber is not particularly
limited, and any fluororubber having a peroxide-cross-linkable moiety may
be used. The peroxide-cross-linkable moiety is not particularly limited,
and examples thereof include moieties containing a propylene (P) unit,
moieties containing iodine atoms, and moieties containing bromine atoms.

[0068] The amount of the cross-linking aid is 0.01 to 10 parts by mass,
and preferably 0.1 to 5.0 parts by mass, relative to 100 parts by mass of
the fluororubber. If the amount of the cross-linking aid is less than
0.01 parts by mass, the cross-linking time tends to be impractically
long. If the amount of the cross-linking aid is more than 10 parts by
mass, the cross-linking time may be too short, and the compression set of
the molded article tends to decrease.

[0069] The polyol cross-linking can be performed when a
polyol-cross-linkable fluororubber and a polyhydroxy compound as the
cross-linking agent are used.

[0070] The polyol-cross-linkable fluororubber is not particularly limited,
and any fluororubber having a polyol-cross-linkable moiety may be used.
The polyol-cross-linkable moiety is not particularly limited, and
examples thereof include moieties having a vinylidene fluoride (VdF)
unit. Examples of the method of introducing the cross-linkable moiety
include a method of copolymerizing cross-linking-site-imparting monomers
when the fluororubber is polymerized.

[0071] As a polyhydroxy compound, a polyhydroxy aromatic compound is
suitably used from the viewpoint of excellent heat resistance.

[0072] The polyhydroxy aromatic compound is not particularly limited, and
examples thereof include 2,2-bis(4-hydroxyphenyl)propane (hereinafter
referred to as bisphenol A), 2,2-bis(4-hydroxyphenyl)perfluoropropane
(hereinafter referred to as bisphenol AF), resorcin,
1,3-dihydroxybenzene, 1,7-dihydroxynaphthalene, 2,7-dihydroxynaphthalene,
1,6-dihydroxynaphthalene, 4,4'-dihydroxydiphenyl, 4,4'-dihydroxystilbene,
2,6-dihydroxyanthracene, hydroquinone, catechol,
2,2-bis(4-hydroxyphenyl)butane (hereinafter referred to as bisphenol B),
4,4-bis(4-hydroxyphenyl)valeric acid,
2,2-bis(4-hydroxyphenyl)tetrafluorodichloropropane,
4,4'-dihydroxydiphenylsulfone, 4,4'-dihydroxydiphenyl ketone,
tri(4-hydroxyphenyl)methane, 3,3',5,5'-tetrachlorobisphenol A, and
3,3',5,5'-tetrabromobisphenol A. These polyhydroxy aromatic compounds may
be metal salts such as alkali metal salts and alkaline earth metal salts,
but these metal salts are preferably not used in the case of coagulating
the copolymer with use of an acid.

[0073] In the case that the cross-linking agent is a polyhydroxy compound,
the cross-linkable fluororubber composition of the present invention
preferably contains a cross-linking accelerator. A cross-linking
accelerator promotes generation of double bonds in molecules in
dehydrofluorination reaction of the main chain of the polymer, and
addition of the polyhydroxy compound to the generated double bonds.

[0074] Examples of the cross-linking accelerator include onium compounds.
Preferable among the onium compounds is at least one selected from the
group consisting of ammonium compounds such as a quaternary ammonium
salt, phosphonium compounds such as a quaternary phosphonium salt,
oxonium compounds, sulfonium compounds, cyclic amines, and monofunctional
amine compounds. Among these, at least one selected from the group
consisting of quaternary ammonium salts and quaternary phosphonium salts
is more preferable.

[0076] The quaternary phosphonium salts are not particularly limited.
Examples thereof include tetrabutylphosphonium chloride,
benzyltriphenylphosphonium chloride (hereinafter referred to as BTPPC),
benzyltrimethylphosphonium chloride, benzyltributylphosphonium chloride,
tributylallylphosphonium chloride, tributyl-2-methoxypropylphosphonium
chloride, and benzylphenyl(dimethylamino)phosphonium chloride. Preferable
among these is benzyltriphenylphosphonium chloride (BTPPC) from the
viewpoints of the excellent cross-linkability and the excellent physical
properties of the molded article.

[0077] The cross-linking accelerator may be a solid solution of a
quaternary ammonium salt and bisphenol AF, a solid solution of a
quaternary phosphonium salt and bisphenol AF, or a chlorine-free
cross-linking accelerator disclosed in JP 11-147891 A.

[0078] The amount of the cross-linking accelerator is preferably 0.01 to 8
parts by mass, and more preferably 0.02 to 5 parts by mass, relative to
100 parts by mass of the fluororubber. If the amount of the cross-linking
accelerator is less than 0.01 parts by mass, cross-linking of the
fluororubber tends not to proceed sufficiently, resulting in a decrease
in the heat resistance and oil resistance of the molded article to be
obtained. If the amount of the cross-linking accelerator is more than 8
parts by mass, molding processability of the cross-linkable fluororubber
composition tends to decrease.

[0079] The polyamine cross-linking can be performed when a
polyamine-cross-linkable fluororubber and a polyamine compound as the
cross-linking agent are used.

[0080] The polyamine-cross-linkable fluororubber is not particularly
limited, and any fluororubber having a polyamine-cross-linkable moiety
may be used. The polyamine-cross-linkable moiety is not particularly
limited, and examples thereof include moieties having a vinylidene
fluoride (VdF) unit. Examples of the method of introducing the
cross-linkable moiety include a method of copolymerizing
cross-linking-site-imparting monomers in polymerization of the
fluororubber.

[0081] Examples of the polyamine compound include hexamethylenediamine
carbamate, N,N'-dicinnamylidene-1,6-hexamethylenediamine, and
4,4'-bis(aminocyclohexyl)methane carbamate. Among these,
N,N'-dicinnamylidene-1,6-hexamethylenediamine is preferable.

[0082] Each of the triazine cross-linking, oxazole cross-linking,
imidazole cross-linking, and thiazole cross-linking can be performed
using a fluororubber cross-linkable in the cross-linking system together
with an oxazole cross-linking agent, imidazole cross-linking agent,
thiazole cross-linking agent, or triazine cross-linking agent.

[0083] Examples of the fluororubbers cross-linkable by these cross-linking
systems include copolymers having copolymerization units derived from the
above cross-linking-site-imparting monomers.

[0084] Examples of the oxazole cross-linking agent, the imidazole
cross-linking agent, the thiazole cross-linking agent, and the triazine
cross-linking agent include compounds including at least two
cross-linkable reactive groups represented by the following formula:

##STR00001##

[0085] (wherein R3s are the same as or different from each other,
each R3 is --NH2, --NHR4, --OH, or --SH, and R4 is a
fluorine atom or a monovalent organic group);

[0086] compounds represented by the following formula:

##STR00002##

[0087] (wherein R5 is --SO2--, --O--, --CO--, a C1 to C6
alkylene group, a C1 to C10 perfluoroalkylene group, or a single bond,
and R6 is either one of the following groups);

##STR00003##

[0088] compounds represented by the following formula:

##STR00004##

(wherein Rf2 is a C1 to C10 perfluoroalkylene group); and

[0089] compounds represented by the following formula:

##STR00005##

(wherein n is an integer of 1 to 10).

[0090] Non-limiting specific examples thereof include
2,2-bis(3,4-diaminophenyl)hexafluoropropane,
2,2-bis[3-amino-4-(N-methylamino)phenyl]hexafluoropropane,
2,2-bis[3-amino-4-(N-ethylamino)phenyl]hexafluoropropane,
2,2-bis[3-amino-4-(N-propylamino)phenyl]hexafluoropropane,
2,2-bis[3-amino-4-(N-phenylamino)phenyl]hexafluoropropane,
2,2-bis[3-amino-4-(N-perfluorophenylamino)phenyl]hexafluoropropane, and
2,2-bis[3-amino-4-(N-benzylamino)phenyl]hexafluoropropane. Among these,
2,2-bis(3,4-diaminophenyl)hexafluoropropane and
2,2-bis[3-amino-4-(N-phenylamino)phenyl]hexafluoropropane are more
preferable from the viewpoints of excellent heat resistance and
particularly good cross-linking reactivity.

[0091] A catalyst for triazine cross-linking can also be used with the
above cross-linking agent. Examples of the catalyst for triazine
cross-linking include organotin compounds such as tetraphenyltin and
triphenyltin. The catalyst for triazine cross-linking may be used alone
without being combined with a cross-linking agent.

[0092] In the case that the fluororubber (A) is a copolymer containing a
copolymerization units derived from cyano group-containing monomers
imparting a cross-linking site, use of an ammonia-producing compound
causes the cyano groups to be cyclotrimerized and thus allows the
triazine cross-linking to proceed. The ammonia-producing compound may be
used alone or in combination with an oxazole cross-linking agent,
imidazole cross-linking agent, thiazole cross-linking agent, or triazine
cross-linking agent. The above ammonia-producing compound is a compound
that generates ammonia at 40° C. to 330° C.

[0093] Preferable examples of the ammonia-producing compound include urea
and ammonium salts. The ammonium salt may be either an organic ammonium
salt or inorganic ammonium salt.

[0094] The urea may be urea or a urea derivative such as biurea, thiourea,
urea hydrochlorides, and biuret.

[0096] Examples of the inorganic ammonium salt include compounds disclosed
in JP 9-111081 A, such as ammonium sulfate, ammonium carbonate, ammonium
nitrate, and ammonium phosphate. Preferable among these is ammonium
phosphate from the viewpoint of vulcanization characteristics.

[0098] Each of these ammonia-producing compounds may be used alone, or two
or more of these may be used in combination.

[0099] The amount of the ammonia-producing compound may be appropriately
adjusted depending on the amount of ammonia to be produced. In general,
the amount thereof is preferably 0.05 to 10 parts by mass, more
preferably 0.1 to 5 parts by mass, and still more preferably 0.2 to 3
parts by mass, relative to 100 parts by mass of the fluororubber. Too
small an amount of the ammonia-producing compound tends to cause a low
cross-linking density, so that the heat resistance and chemical
resistance tend to be insufficient for practical use. In contrast, too
large an amount thereof may cause scorch, so that the storage stability
tends to be poor and the color of the molded article tends not to be
clear.

[0100] The above irradiation cross-linking system is a cross-linking
system in which cross-linking starts upon radiation of active energy rays
such as ultraviolet rays and radiation rays. In this case, a
cross-linking aid such as a polyfunctional unsaturated compound may be
used. The above irradiation cross-linking system is suitable in the case
that the fluororubber is a TFE/P copolymer.

[0101] Examples of the polyfunctional unsaturated compound include
polyfunctional compounds having an ethylenic unsaturated linking group
such as CH2═CH--, CH2═CHCH2--, CF2═CF--,
and --CH═CH--. Particularly, oxime nitroso compounds,
di(meth)acrylate compounds, triester compounds, triallyl isocyanurate
compounds, and polybutadiene compounds are preferable from the viewpoint
of high cross-linking efficiency. Each of these may be used alone or two
or more of these may be used in combination.

[0102] Examples of the oxime nitroso compound include dinitroso benzene.
Examples of the di(meth)acrylate compound include NK Ester 9G (product of
Shin-Nakamura Chemical Co., Ltd.). Examples of the triester compound
include Hi-Cross M (product of Seiko Chemical Co., Ltd.) and NK Ester
TMTP (product of Shin-Nakamura Chemical Co., Ltd.). Examples of the
triallyl isocyanurate compound include triallyl isocyanurate (TAIC) and
trimethallyl isocyanurate (TMAIC). Examples of the polybutadiene compound
include NISSO-PB (product of Nippon Soda Co., Ltd.). Among these,
triallyl isocyanurate (TAIC) is suitable from the viewpoint of high
cross-linking efficiency.

[0103] The addition amount (blending amount) of the polyfunctional
unsaturated compound is preferably 0.1 to 20 parts by mass relative to
100 parts by mass of the fluororubber. Such an amount leads to a further
increase in the cross-linking efficiency. The lower limit is more
preferably 0.5 parts by mass, and still more preferably 1 part by mass,
while the upper limit is more preferably 10 parts by mass and still more
preferably 5 parts by mass.

(B) Fluororesin

[0104] The fluororesin (B) is preferably a fluorine-containing ethylenic
polymer containing a structural unit derived from at least one
fluorine-containing ethylenic monomer, and also preferably a
melt-processable fluororesin. Examples of the fluorine-containing
ethylenic monomer include one or two or more perfluoroolefins such as
tetrafluoroethylene [TFE] and a perfluoroethylenic unsaturated compound
represented by formula (21):

(wherein X2 represents a hydrogen atom or a fluorine atom, X3
represents a hydrogen atom, a fluorine atom, or a chlorine atom, and n
represents an integer of 1 to 10).

[0105] The fluororesin (B) may be a fluorine-containing ethylenic polymer
having a structural unit derived from a monomer copolymerizable with the
above fluorine-containing ethylenic monomer. Examples of such a monomer
include non-fluorinated ethylenic monomers other than the above
perfluoroolefin and fluoroolefin. Examples of the non-fluorinated
ethylenic monomer include ethylene, propylene, and alkyl vinyl ethers.
Here, the alkyl vinyl ether refers to an alkyl vinyl ether having a C1 to
C5 alkyl group.

[0106] Among these, the following fluoropolymers are preferable from the
viewpoint of a good effect of reducing the coefficient of friction of the
fluororubber molded articles.

(1) Ethylene/TFE copolymer [ETFE] (2) Copolymer of TFE and one or two or
more perfluoroethylenic unsaturated compounds represented by formula
(21):

CF2═CF--Rf3 (21)

(wherein Rf3 represents --CF3 or --ORf4, and
Rf4 represents a C1 to C5 perfluoroalkyl group), such as
TFE/perfluoro(alkyl vinyl ether) [PAVE] copolymer [PFA] or
TFE/hexafluoropropylene [HFP] copolymer [FEP] (3) Copolymer of TFE, VdF,
and one or two or more perfluoroethylenic unsaturated compounds
represented by formula (21):

[0107] The fluororesin is more preferably at least one selected from the
group consisting of ETFE, FEP, PFA, a TFE/VdF/HFP copolymer, PVdF, and a
CTFE/TFE copolymer, still more preferably at least one selected from the
group consisting of ETFE, FEP, PFA, and a CTFE/TFE copolymer,
particularly preferably at least one selected from the group consisting
of ETFE, FEP, and a CTFE/TFE copolymer, and most preferably FEP from the
viewpoint of particularly excellent compatibility with the fluororubber
(A).

ETFE

[0108] ETFE is preferable from the viewpoint of an increase in the
mechanical properties and fuel barrier properties of the fluororubber
molded articles. The molar ratio of the TFE unit to the ethylene unit is
preferably 20:80 to 90:10, more preferably 37:63 to 85:15, and
particularly preferably 38:62 to 80:20.

[0109] ETFE may be a copolymer of TFE, ethylene, and a monomer
copolymerizable with TEE and ethylene. Examples of the copolymerizable
monomer include monomers represented by the following formulas
CH2═CX4Rf5, CF2═CFRf5,
CF2═CFORf5, and CH2═C(Rf5)2
(wherein X4 represents a hydrogen atom or a fluorine atom, and
Rf5 represents a fluoroalkyl group which may contain an
ether-bind-forming oxygen atom). Among these, fluorine-containing vinyl
monomers represented by CH2═CX4Rf5 are
preferable, and fluorine-containing vinyl monomers represented by
CH2═CX4Rf5 in which Rf5 is a C1 to C8
fluoroalkyl group are more preferable.

[0111] The monomer copolymerizable with TFE and ethylene may be an
aliphatic unsaturated carboxylic acid such as itaconic acid and itaconic
acid anhydride.

[0112] The amount of the monomer copolymerizable with TFE and ethylene is
preferably 0.1 to 10 mol %, more preferably 0.1 to 5 mol %, and
particularly preferably 0.2 to 4 mol % of the amount of the
fluorine-containing ethylenic polymer.

FEP

[0113] FEP is preferable from the viewpoint of particularly excellent heat
resistance and excellent fuel barrier properties of the fluororubber
molded article. FEP is not particularly limited, and is preferably a
copolymer of 70 to 99 mol % of the TFE unit and 1 to 30 mol % of the HFP
unit, and more preferably a copolymer of 80 to 97 mol % of the TFE unit
and 3 to 20 mol % of the HFP unit. If the amount of the TFE unit is less
than 70 mol %, the mechanical properties tend to decrease. If the amount
thereof is more than 99 mol %, the melting point increases too high and
thus the moldability tends to decrease.

[0114] FEP may be a copolymer of TFE, HFP and a monomer copolymerizable
with TFE and HFP. Examples of the monomer include perfluoro(alkyl vinyl
ether) [PAVE] represented by CF2═CF--ORf6 (wherein
Rf6 represents a C1 to C5 perfluoroalkyl group), vinyl monomers
represented by CX8X6═CX7(CF2)nX8
(wherein X5, X6, and X7 are the same as or different from
each other and each of these is a hydrogen atom or a fluorine atom,
X8 represents a hydrogen atom, a fluorine atom, or a chlorine atom,
and n represents an integer of 2 to 10), and alkyl perfluorovinyl ether
derivatives represented by CF2═CF--OCH2--Rf7 (wherein
Rf7 represents a C1 to C5 perfluoroalkyl group). Among these, PAVE
is preferable.

[0115] The PAVE is preferably at least one selected from the group
consisting of perfluoro(methyl vinyl ether) [PMVE], perfluoro(ethyl vinyl
ether) [PEVE], perfluoro(propyl vinyl ether) [PPVE], and perfluoro(butyl
vinyl ether), and is more preferably at least one selected from the group
consisting of PMVE, PEVE, and PPVE.

[0116] The alkyl perfluorovinyl ether derivative is preferably one in
which Rf7 is a C1 to C3 perfluoroalkyl group, and more preferably
CF2═CF--OCH2--CF2CF3.

[0117] FEP preferably has 0.1 to 10 mol % of the monomer unit derived from
a monomer copolymerizable with TFE and HFP, and 90 to 99.9 mol % of the
TFE unit and the HFP unit in total. If the amount of the copolymerizable
monomer unit is less than 0.1 mol %, the moldability,
environmental-stress-cracking resistance, and stress cracking resistance
tend to deteriorate. If the amount is more than 10 mol %, the low
chemical permeability, heat resistance, mechanical properties, and
productivity tend to deteriorate.

PFA

[0118] PFA is preferable from the viewpoints of excellent heat resistance
and excellent fuel barrier properties of the fluororubber molded article.
PFA is not particularly limited, and is preferably a copolymer of 70 to
99 mol % of the TFE unit and 1 to 30 mol % of the PAVE unit, and more
preferably a copolymer of 80 to 97 mol % of the TFE unit and 3 to 20 mol
% of the PAVE unit. If the amount of the TFE unit is less than 70 mol %,
the mechanical properties tend to decrease. If the amount is more than 99
mol %, the melting point is too high and thus the moldability tends to
decrease.

[0119] The PAVE is preferably at least one selected from the group
consisting of perfluoro(methyl vinyl ether) [PMVE], perfluoro(ethyl vinyl
ether) [PEVE], perfluoro(propyl vinyl ether) [PPVE], and perfluoro(butyl
vinyl ether), more preferably at least one selected from the group
consisting of PMVE, PEVE, and PPVE, and still more preferably PMVE.

[0120] PFA may be a copolymer of TFE, PAVE, and a monomer copolymerizable
with TFE and PAVE. Examples of the monomer include HFP, vinyl monomers
represented by CX5X6═CX7(CF2)nX8
(wherein X5, X6 and X7 are the same as or different from
each other, and each of these is a hydrogen atom or a fluorine atom,
X8 represents a hydrogen atom, a fluorine atom, or a chlorine atom,
and n represents an integer of 2 to 10), and alkyl perfluorovinyl ether
derivatives represented by CF2═CF--OCH2--Rf7 (wherein
Rf7 represents a C1 to C5 perfluoroalkyl group).

[0121] The alkyl perfluorovinyl ether derivative is preferably one in
which Rf7 is a C1 to C3 perfluoroalkyl group, and more preferably
CF2═CF--OCH2--CF2CF3.

[0122] PFA preferably has 0.1 to 10 mol % of the monomer unit derived from
a monomer copolymerizable with TFE and PAVE, and 90 to 99.9 mol % of the
TFE unit and the PAVE unit in total. If the amount of the copolymerizable
monomer unit is less than 0.1 mol %, the moldability,
environmental-stress-cracking resistance, and stress cracking resistance
tend to deteriorate. If the amount is more than 10 mol %, the low
chemical permeability, heat resistance, mechanical properties, and
productivity tend to deteriorate.

CTFE/TFE Copolymer

[0123] The CTFE/TFE copolymer preferably has a molar ratio of the CTFE
unit to the TFE unit CTFE:TFE of 2:98 to 98:2, and more preferably 5:95
to 90:10. If the amount of the CTFE unit is less than 2 mol %, the
chemical permeability tends to decrease and melt processing tends to be
difficult. If the amount of the CTFE unit is more than 98 mol %, heat
resistance and chemical resistance in molding may decrease.

[0124] The CTFE/TFE copolymer may be a copolymer of CTFE, TFE, and a
monomer copolymerizable with CTFE and TFE. Examples thereof include
ethylene, VdF, HFP, perfluoro(alkyl vinyl ether) [PAVE] represented by
CF2═CF--ORf6 (wherein Rf6 represents a C1 to
C5 perfluoroalkyl group), vinyl monomers represented by
CX5X6═CX7(CF2)nX8 (wherein X5,
X6, and X7 are the same as or different from each other and
each of these is a hydrogen atom or a fluorine atom, X8 represents a
hydrogen atom, a fluorine atom, or a chlorine atom, and n represents an
integer of 2 to 10), and alkyl perfluorovinyl ether derivatives
represented by CF2═CF--OCH2--Rf7 (wherein Rf7
represents a C1 to C5 perfluoroalkyl group). Among these, PAVE is
preferable.

[0125] The PAVE is preferably at least one selected from the group
consisting of perfluoro(methyl vinyl ether) [PMVE], perfluoro(ethyl vinyl
ether) [PEVE], perfluoro(propyl vinyl ether) [PPVE], and perfluoro(butyl
vinyl ether). Among these, at least one selected from the group
consisting of PMVE, PEVE, and PPVE is more preferable.

[0126] The alkyl perfluoro vinyl ether derivative is preferably one
wherein Rf7 is a C1 to C3 perfluoroalkyl group, and more preferably
CF2═CF--OCH2--CF2CF3.

[0127] The CTFE/TFE copolymer preferably has 0.1 to 10 mol % of the
monomer unit derived from a monomer copolymerizable with CTFE and TFE,
and 90 to 99.9 mol % of the CTFE unit and the TFE unit in total. If the
amount of the copolymerizable monomer unit is less than 0.1 mol %, the
moldability, environmental-stress-cracking resistance, and stress
cracking resistance tend to deteriorate. If the amount is more than 10
mol %, the low chemical permeability, heat resistance, mechanical
properties, and productivity tend to deteriorate.

[0128] The fluororesin (B) preferably has a melting point of 120°
C. to 340° C., more preferably 150° C. to 330° C.,
and still more preferably 170° C. to 320° C. If the melting
point of the fluororesin (B) is lower than 120° C., bleed out
tends to occur in cross-linking and molding. If the melting point is more
than 340° C., mixing of the fluororesin (B) with the fluororubber
(A) tends to be difficult.

[0129] The fluororesin (B) preferably has a melt flow rate [MFR] of 0.1 to
100 g/10 min. Too low an MFR may cause inferior abrasion resistance, and
too high an MFR may cause difficulties in molding.

[0130] To increase the compatibility between the fluororesin (B) and the
fluororubber (A), at least one polyfunctional compound may be added. A
polyfunctional compound is a compound having at least two functional
groups which are structurally the same as or different from each other in
one molecule.

[0131] Each of the functional groups in the polyfunctional compound may be
any functional group generally known to have reactivity, such as
carbonyl, carboxyl, haloformyl, amide, olefin, amino, isocyanate,
hydroxy, and epoxy. A compound having these functional groups not only
has high compatibility with the fluororubber (A) but also is reactive to
functional groups known to have reactivity in the fluororesin (B), and is
therefore expected to further increase the compatibility.

[0132] The fluororubber (A) preferably constitute 45 to 97% by mass of the
composition. Too small an amount of the fluororubber may not lead to a
fluororubber molded article having characteristics as a rubber, while too
large an amount of the fluororubber may not lead to a fluororubber
product having low friction properties.

[0133] The cross-linkable fluororubber composition of the present
invention preferably has a mass ratio (A)/(B) of the fluororubber (A) to
the fluororesin (B) of 60/40 to 97/3. Too small an amount of the
fluororesin (B) may lead to an insufficient effect of reducing the
coefficient of friction, while too large an amount of the fluororesin (B)
may notably deteriorate the rubber elasticity, which may lead to loss of
flexibility. From the viewpoint of good flexibility and good low
frictional properties, the ratio (A)/(B) is more preferably 65/35 to
95/5, and still more preferably 70/30 to 90/10.

[0134] If necessary, the cross-linkable fluororubber composition of the
present invention may further contain common formulation ingredients
including additives for fluororubber such as fillers, processing aids,
plasticizers, colorants, stabilizers, bonding aids, release agents,
electric conductivity imparting agents, thermal conductivity imparting
agents, surface non-adhesive agents, flexibility imparting agents, heat
resistance improvers, and flame retardants, to the extent that the
effects of the present invention are not deteriorated.

[0135] The cross-linkable fluororubber composition of the present
invention does not contain a fluorine-containing thermoplastic elastomer.

[0136] The method for producing the fluororubber molded article of the
present invention will be described below.

[0137] The method for producing the fluororubber molded article of the
present invention includes

[0138] (I) a step of obtaining a cross-linkable fluororubber composition
by co-coagulating the fluororubber (A) and the fluororesin (B);

[0139] (II) a molding and cross-linking step of obtaining a cross-linked
molded article by molding and cross-linking the cross-linkable
fluororubber composition; and

[0140] (III) a heat-treatment step of obtaining a fluororubber molded
article by heating the cross-linked molded article at a temperature not
lower than a melting point of the fluororesin (B).

[0141] Hereinafter, the respective steps are described.

Step (I)

[0142] This step is for obtaining a cross-linkable fluororubber
composition by co-coagulating the fluororubber (A) and the fluororesin
(B).

[0143] Examples of the method of co-coagulation include

[0144] (i) performing coagulation after mixing an aqueous dispersion of
the fluororubber (A) and an aqueous dispersion of the fluororesin (B);

[0145] (ii) performing coagulation after adding the powder of the
fluororubber (A) to the fluororesin (B); and

[0146] (iii) performing coagulation after adding the powder of the
fluororesin (B) to an aqueous dispersion of the fluororubber (A).

[0147] The method for the co-coagulation is preferably the method (i)
particularly from the viewpoint of easy uniform dispersion of the
respective resins.

[0148] The coagulation by the above coagulation methods (i) to (iii) can
be performed using a coagulant, for example. Such a coagulant is not
particularly limited, and examples thereof include known coagulants such
as aluminum salts (e.g. aluminum sulfate, alum), calcium salts (e.g.
calcium sulfate), magnesium salts (e.g. magnesium sulfate), and
monovalent cation salts (e.g. sodium chloride, potassium chloride). In
the case of performing coagulation using a coagulant, an acid or an
alkali may be added to adjust the pH for promotion of the coagulation.

[0149] Since a cross-linking agent is required depending on the
cross-linking system of the fluororubber, the step (I) may also
preferably be a step of co-coagulating the fluororubber (A) and the
fluororesin (B) to obtain co-coagulation powder, and thereafter mixing
the co-coagulation powder and a cross-linking agent to obtain a
cross-linkable fluororubber composition.

[0150] The mixing of the co-coagulation powder and a cross-linking agent
can be performed by a conventionally known method. For example, the
mixing may be performed using an open roll under temperature and time
conditions which allow the co-coagulation powder and the cross-linking
agent to be sufficiently mixed.

Molding and Cross-Linking Step (II)

[0151] This step is a step of producing a cross-linked molded article by
molding and cross-linking the cross-linkable fluororubber composition
obtained in the mixing step (I). The order of molding and cross-linking
is not limited; that is, cross-linking may be performed after molding,
molding may be performed after cross-linking, or molding and
cross-linking may be performed at the same time.

[0152] In the case of a hose or a long plate, for example, a method of
performing extrusion-molding and then cross-linking is appropriately
employed. In the case of products having other shapes, a method can be
employed in which block-shaped cross-linked product is first obtained and
then the product is subjected to molding processes such as cutting. In
the case of comparatively simple molded articles such as piston rings and
oil seals, a method of performing molding and cross-linking with a mold
at the same time is also commonly employed.

[0153] Examples of the molding method include, but not limited to,
extrusion-molding, pressure-molding using a mold, and injection-molding.

[0154] The cross-linking method may also be steam cross-linking,
pressure-molding, irradiation cross-linking, and a common method in which
the cross-linking reaction is initiated by heating. In the present
invention, the cross-linking reaction initiated by heating is suitable
from the viewpoint of smooth migration of the fluororesin to the surface
layer of the cross-linkable fluororubber composition.

[0155] The method for molding and cross-linking a cross-linkable
fluororubber composition and the conditions thereof may be within the
scope of known methods and conditions of the molding and cross-linking.

[0156] The non-limiting specific cross-linking conditions may be
appropriately determined usually from a temperature range of 150°
C. to 300° C. and a cross-linking time of 1 minute to 24 hours,
according to the kind of the cross-linking agent and the like to be used.

[0157] The cross-linking of rubber sometimes includes a post treatment
step called secondary cross-linking after the first cross-linking
treatment (called primary cross-linking). The conventional secondary
cross-linking, as described in the heat-treatment step (III) below, is a
treatment step different from the molding and cross-linking step (II) and
heat-treatment step (III) in the present invention.

Heat-Treatment Step (III)

[0158] In this step, the cross-linked molded article obtained in the
molding and cross-linking step (II) is heated to a temperature higher
than the melting point of the fluororesin (B) so as to obtain a
fluororubber molded article.

[0159] The heat-treatment step (III) in the present invention is a
treatment step for increasing the proportion of the fluororesin on the
cross-linked molded article surface. To increase the proportion, the
heating temperature is employed which is not lower than the melting point
of the fluororesin (B) and is lower than the pyrolysis temperatures of
the fluororubber (A) and the fluororesin (B).

[0160] If the heating temperature is lower than the melting point, the
fluororesin proportion on the cross-linkable molded product surface does
not increase sufficiently. In order to avoid the pyrolysis of the
fluororubber and the fluororesin, the heating temperature must be lower
than the lower of the pyrolysis temperatures of the fluororubber (A) and
the fluororesin (B). The heating temperature is preferably higher than
the melting point of the fluororesin by 5° C. or more from the
viewpoint of easy reduction of the friction in a short time.

[0161] The above upper limit of the temperature is for common
fluororubbers. The upper limit of the temperature is not limited thereto
for fluororubbers having super heat resistance because the upper limit of
the temperature in this case is the pyrolysis temperature of the
fluororubber having super heat resistance.

[0162] In the heat-treatment step (III), the heating temperature is
closely related to heating time, and the heating time is preferably
comparatively long at temperatures comparatively close to the lower limit
of the temperature, and is preferably comparatively short at temperatures
comparatively close to the upper limit of the temperature. As above, the
heating time may be appropriately set depending on the relation with the
heating temperature, and is substantially up to 48 hours except for the
case of using a highly heat-resistant fluororubber because very long
heating treatment sometimes causes heat deterioration of the
fluororubber. Usually, the heat-treatment time is preferably 1 minute to
48 hours, and more preferably 1 minute to 24 hours from the viewpoint of
good productivity, but is preferably 24 hours to 48 hours in order to
sufficiently decrease the coefficient of friction.

[0163] The present inventors are the first to find the phenomenon of the
increase in the proportion of the fluororesin in the surface region of
the cross-linked molded article in the heat-treatment step (III).

[0164] The secondary cross-linking conventionally performed is a treatment
of completely decomposing the cross-linking agent remaining after the end
of the primary cross-linking so as to complete cross-linking of the
fluororubber, thereby increasing the mechanical properties and
compression set characteristics of the cross-linked molded article.

[0165] Accordingly, in the conventional secondary cross-linking
conditions, coexistence of the fluororesin (B) is not expected. Even if
the above cross-linking conditions incidentally overlap the heating
conditions in the heat-treatment step in the present invention, the
heating conditions are set within the scope of the purpose of completing
cross-linking of the fluororubber (completing decomposition of the
cross-linking agent) without consideration of the existence of the
fluororesin as a factor in setting of the cross-linking conditions.
Hence, the heating under the above conditions do not lead to the
condition of heat-softening or fusing the fluororesin (B) added in the
cross-linked rubber product (not uncross-linked rubber product).

[0166] The secondary cross-linking for completing the cross-linking of the
fluororubber (A) (completely decomposing the cross-linking agent) may be
performed in the molding and cross-linking step (II) in the present
invention.

[0167] The remaining cross-linking agent may be decomposed and thus
cross-linking of the fluororubber (A) may be completed in the
heat-treatment step (III); still, this cross-linking of the fluororubber
(A) in the heat-treatment step (III) is merely a secondary effect.

[0168] The production method of the present invention enables to obtain a
fluororubber molded article having significantly better characteristics
of the fluororesin, such as low friction properties, non-adhesiveness,
water repellency, and oil repellency, than ones which are not
heat-treated. The fluororubber molded article produced by the production
method of the present invention actually will also have the
characteristics of the fluororubber at portions other than the surface
region, and have, overall, low friction properties, non-adhesiveness,
water repellency, oil repellency, and elastomeric properties in a
balanced manner. Further, the fluororubber molded article does not have
definite interface between the fluororesin and the fluororubber, and
therefore the region rich in the fluororesin on the surface does not come
off or peel off.

[0169] Hence, the fluororubber molded article has higher durability than
conventional fluororubber molded articles having fluororesin applied or
adhered to the fluororubber surface.

[0170] The fluororubber molded article of the present invention is useful
as a product such as a sealing material, a slide member, and a
non-adhesive member, owing to the low friction properties,
non-adhesiveness, water repellency and oil repellency (high contact
angle).

[0171] Examples thereof include, but not limited to, the following molded
articles.

[0174] In the airplane, rocket and shipbuilding fields, examples of the
sealing material include diaphragms, O (square)-rings, valves, packings,
and other various sealing materials, and these can be used in fuel
systems. Specifically, in the airplane field, the molded articles are
used as jet engine valve stem seals, gaskets and O-rings, rotating shaft
seals, hydraulic gaskets and fire wall seals and the like; in the
shipbuilding field, the molded articles are used as screw propeller shaft
stern seals, diesel engine suction and exhaust valve stem seals,
butterfly valve seals, butterfly valve shaft seals and the like.

[0175] Examples of the sealing materials in the chemical plant field
include valves, packings, diaphragms, O (square)-rings, and other various
sealing materials, and these can be used in various steps of producing
chemicals such as medicinal chemicals, agrochemicals, paints and resins.
More specifically, the molded articles can be used as seals in chemical
pumps, flowmeters and piping systems, heat exchanger seals, glass cooler
packings in sulfuric acid production plants, seals in agrochemical
spreaders and agrochemical transfer pumps, gas piping seals, plating bath
seals, high-temperature vacuum drier packings, papermaking belt roller
seals, fuel cell seals, wind tunnel joint seals, tube joining part
packings in gas chromatographs and pH meters, and seals, diaphragms and
valve parts in analytical apparatus and physical and chemical apparatus.

[0176] In the photographic field (e.g. developing machines), the printing
field (e.g. printing machines) and the painting field (e.g. painting
equipment), the molded articles can be used for example as seals and
valve parts in dry-process copying machines.

[0177] In the food industry plant equipment field, examples of the sealing
material include valves, packings, diaphragms, O (square)-rings and
various sealing materials, and these can be used in food production
steps. More specifically, the molded articles can be used as plate type
heat exchanger seals, and vending machine electromagnetic valve seals.

[0178] In the nuclear power plant equipment field, examples of the sealing
material include packings, O-rings, diaphragms, valves, and various seal
members.

[0179] In the general industry field, examples of the sealing material
include packings, O-rings, diaphragms, valves, and various seal members.
More specifically, the molded articles are used as seals and bearing
seals in hydraulic and lubricating systems, windows and other seals in
dry cleaning equipment, uranium hexafluoride enrichment apparatus seals,
seal (vacuum) valves in cyclotrons, automatic packaging machine seals,
diaphragms in pumps (in pollution-monitoring apparatus) for analyzing
sulfurous acid gas and chlorine gas in air, and the like.

[0180] In the electric system field, the molded articles are specifically
used as bullet train (Shinkansen) insulating oil caps, liquid-sealed
transformer benching seals and the like.

[0181] In the fuel cell field, the articles are specifically used as seal
materials between electrodes and a separator and as seals in hydrogen,
oxygen or product water piping systems.

[0182] In the electronic component field, the articles are specifically
used as radiator materials, electromagnetic wave shield materials,
computer hard disk drive gaskets and the like.

[0183] Those sealing materials which can be used in situ molding are not
particularly limited, and examples thereof include engine oil pan
gaskets, gaskets for magnetic recording apparatus, and clean room filter
unit sealing materials.

[0184] The molded articles can be particularly suitably used as gaskets
for magnetic recording apparatus (hard disk drives) and sealing materials
for clean equipment such as sealing materials in semiconductor
manufacturing apparatus or storehouses for wafers or other devices.

[0185] Further, the molded articles are particularly suitably used as
sealing materials for fuel cells, such as packings used between fuel cell
electrodes or in peripheral piping systems.

[0187] Generally, the examples include fluororubber products used as parts
that slide in contact with other materials.

Non-Adhesive Members:

[0188] Examples of the sealing material in the computer field include hard
disk crash stoppers.

[0189] Fields utilizing water repellency and oil repellency: Examples of
the sealing material include automobile wiper blades, and coated fabrics
for outdoor tents.

EXAMPLES

[0190] The following examples illustrate the present invention more
specifically. These examples are, however, by no means limitative of the
scope of the present invention.

[0191] The physical characteristics reported herein were measured by the
following methods.

(1) Cross-Linking (Vulcanization) Characteristics

[0192] The minimum torque (ML), the maximum torque (MH), the induction
time (T10), and the optimum vulcanization time (T90) were measured with a
Curelastometer Type II (product of JSR Corporation).

(2) 100% Modulus (M100)

[0193] The measurement was performed in accordance with JIS K6251.

(3) Tensile Strength at Break (Tb)

[0194] The measurement was performed in accordance with JIS K6251.

(4)Elongation at Break (Eb)

[0195] The measurement was performed in accordance with JIS K6251.

(5) Hardness (Shore A)

[0196] The measurement was performed with Durometer Type A in accordance
with JIS K6253 (peak value).

(6) Coefficient of Friction

[0197] The measurement was performed with Friction Player FPR2000 (product
of Rhesca Corporation) under the conditions of a weight of 20 g, rotation
mode, the number of rotations of 60 rpm, and the radius of rotation of 10
mm. In a stabilized state five minutes or longer after the rotation, the
coefficient of friction was read and the value was taken as the
coefficient of dynamic friction.

[0198] The used materials mentioned in the tables and herein are the ones
listed below.

[0204] A dispersion (polymer content: 24% by mass) of the fluororubber (A)
and a dispersion (polymer content: 32% by mass) of the fluororesin (B)
were mixed such that the solids content ratio of the fluororubber (A) to
the fluororesin (B) was 70:30. The resulting mixture was named dispersion
D.

[0205] Next, 400 g of the dispersion D was added to 500 g of pure water.
The mixture was further mixed with 2 g of aluminum sulfate while being
stirred with a mixer. The mixture was stirred for about three minutes and
the solids content was removed.

[0206] The solids content was dried at 80° C. in an oven for 20
hours. The resulting product is a co-coagulum.

[0207] The obtained co-coagulum was placed around two 8-inch rolls in an
open roll, and 14 parts by mass of the filler and 0.3 parts by mass of
the ammonia-producing compound were added to 100 parts by mass of the
co-coagulum. The obtained mixture was kneaded for 20 minutes. The mixture
was then cooled for 24 hours, and kneaded again at 30° C. to
80° C. for 20 minutes using the open roll provided with two 8-inch
rolls, and thereby a full compound (cross-linkable fluororubber
composition) was prepared.

[0208] The cross-linking (vulcanization) characteristics of the full
compound were determined. Table 1 shows the results.

Molding and Cross-Linking Step (II)

(Molding Step)

[0209] The obtained full compound was molded with an 8-inch open roll into
an un-cross-linked fluororubber sheet which eventually had a thickness of
3 mm.

(Cross-Linking Step)

[0210] The un-cross-linked fluororubber sheet was press-cross-linked with
a mold at 180° C. for 15 minutes, and thereby a cross-linked
fluororubber sheet having a thickness of 2 mm was obtained.

[0214] A cross-linked fluororubber sheet was produced through the same
molding and cross-linking step as that in Example 1, except that the
fluororubber (A) was not co-coagulated with a fluororesin to form a
cross-linkable fluororubber composition, the amount of the filler was
changed to 20 parts by mass, and the amount of the ammonia-producing
compound was changed to 0.4 parts by mass.

[0215] The obtained cross-linked fluororubber sheet was placed in a
furnace maintained at 250° C. for 48 hours to be heated as in
Example 1.

[0217] A cross-linked fluororubber sheet was produced through the same
molding and cross-linking step as that in Example 1, except that the
fluororubber (A) and the fluororesin (B) were kneaded at a material
temperature of 200° C., the amount of the filler was changed to 20
parts by mass, and the amount of the ammonia-producing compound was
changed to 0.3 parts by mass.

[0218] The obtained cross-linked fluororubber sheet was placed in a
furnace maintained at 250° C. for 48 hours to be heated as in
Example 1.

[0221] The results in Table 1 and Table 2 show that a fluororubber molded
article having excellent mechanical strength and low coefficient of
friction can be obtained by heat-treating a cross-linkable fluororubber
compound (full compound) prepared under the specific conditions at a
temperature higher than the melting point of the fluororesin.

INDUSTRIAL APPLICABILITY

[0222] The fluororubber molded article of the present invention can be
used as a sealing material, a slide member, and a non-adhesive member.